Tuesday, 16 January 2018

Streaky Box Elder a mixed up and unwanted plant

This is one confusing plant. Not just the striking - or shocking if you view variegation the way I do - albino streaks through every part of the normally green bits of the tree. But its identity and its name.

These fruits are the typical winged fruit of a maple, or Acer. But we all know maples have those lovely hand-like (palmate) leaves. Well, most of them (particularly in North America) do but this one doesn't.

The leaves are divided into 'leaflets', very like an ash tree or perhaps an elder. Sure enough, the common names include Box Elder, which references its odd leaves (and, I gather, to the wood being reminiscent of a Boxwood, Buxus), and Ash-leaved Maple, which references ... well, the ash, Fraxinus.

Acer negundo, as we botanists call it, is tough old species. We have this giant old one near the Terrace Tea Rooms in Melbourne Gardens, most of it living up the cultivar name of 'Variegatum', and the modified common name, Variegated Box Elder.

The species name, negundo, is another nod to look-alike plant, Vitex negundo. That plant also has leaves divided into leaflets, five in its case, and was named after a local Sanskrit/Bengali word for the plant, nirgudi.

The Box Elders have three to seven, or rarely nine, leaflets. A handful of varieties have been described based on the number of leaflets, how hairy they are, and the colour of the stems (some are a beautiful glaucous grey). Ours seems to match the variant found throughout most of the USA called variety negundo.

So much for its name. Those white streaks and blotches bother me in the leaves but on the early summer day when I took these pictures, I have to say I was quite taken by the veils of white fruits. That's cool. And no doubt responsible for another of this cultivar's common names, Ghost Maple.

No sooner do I find myself attracted to a variegated plant though and it shows me its flaws. This one is reverting to green foliage and fruits in various places. Which begs the obvious question: will the entire plant eventually become green-washed?

Variegation you'll recall is mostly likely the result of a genetic mutation. It seems that in this specimen the 'defect' holds true mostly but ever now and then reverts back to normal at a growing tip - so one branch becomes green again. What you can't tell from a single observation like this is whether the tree is transitioning - if I can use that term - to no variegation, or simply expressing a similar proportion of reversion to what it's done all its life.

They say this cultivar is one of the most commonly grown in Australia and you do see planted as a street tree not far from the Melbourne Gardens. So I could soon make my own assessment as to whether there more all-green leaves on older trees or whether they are in roughly the same proportion no matter what the age of the tree.That all assumes of course there is no manipulation of the system, which it turns out there is.

In VicFlora, where the species is listed as a 'naturalised' species for the State (i.e. a weed that has become established among natural vegetation) we read that 'variegated forms soon revert to wholly green-leaved plants in the wild'. But, 'in cultivation, green-leaved sucker stems are usually removed to retain the variegated foliage of the central stem'. These statements are borrowed from the Horticultural Flora of South-eastern Australia.

The sucker stems in Melbourne Gardens are all over the tree, as they are in the street trees nearby, and it seems their days are numbered. Otherwise our Variegated Box Elder would simply be a Box Elder.

Given the rather disparaging remarks made about the potential of this species as a weed of forests and woodlands, particularly near rivers, perhaps instead of removing green shoots we should remove the whole tree. In Victoria Box Elder is already 'a problem at Wilsons Reserve in Ivanhoe and in Knox City Council'. 'In fact' says the Queensland-based Weeds of Australia site, 'some environmentalists believe it to be among the worst environmental weeds growing along waterways in Melbourne'.

This is where our carefully manicured Variegated Box Elder saves the day. According to the Horticultural Flora it produces sterile seed. Lots of it, but none perpetuating the parent. I'm assuming this refers to the variegated-leaf branches only, providing another incentive to trim the green growth. The choice seems to be between a weedy green tree and a barren but benign ghost-like mutant.